As I noted in another response, you can't cast a rectangular prism and expect a good result. You'd need to add some angle to the edges to ensure it solidifies correctly, but doing so takes away from the monolith feel.

That's also a lot of polishing for the finish you would get from sand casting.

There is a video of it, it has a triangular cross section.

There appear to be screw holes (or rivets), so I'm going to guess that it's three aluminium sheets screwed onto an interior scaffold.

https://www.instagram.com/p/CH_iU8JgeIr/